This invention generally relates to thermal stressing of electrical components. More particularly, it relates to an apparatus and method for thermally stressing by rapidly cooling of electrical components of high thermal mass emerging from the hot zones of a continuous furnace, or belt furnace.
Currently, a sample of components from selected lots is generally sacrificed for thermal stress tests, and if the sample passes, the associated lots are candidates for shipment to customers. Normal production electronic components are not thermally stressed. However, reliability requirements are increasing and there is interest in providing thermal stress to normal production electronic components to remove those with a thermally sensitive class of manufacturing defects.
To find manufacturing defects which would lead to early life fails, high thermal mass semiconductor modules are provided a rapid thermal stress, by first rapid heating and then rapid cooling. Parts are conventionally thermally stressed by methods including rapid immersion in high and low temperature fluids and batch ovens.
The batch ovens are useful for evaluating resistance to thermal stress because they provide excellent temperature and ambient gas control, uniform heating of the entire component and the capability for automatic, repeated heating and cooling cycles covering a wide range from several hundred degrees to liquid nitrogen temperature. However, batch ovens do require a significant stabilization period after oven load to provide all of these characteristics.
Heating coils are used in a batch oven to rapidly raise the temperature. To rapidly lower the temperature, cold nitrogen is directed to flow through the oven. Available batch ovens have important advantages for providing thermal stress: they provide excellent temperature and ambient gas control (after a stabilization period when the oven is loaded), uniform heating and cooling of the entire component, and the capability for heating and cooling cycles covering a wide range from several hundred degrees to the temperature of liquid nitrogen or liquid helium. The ovens can be programmed to repeat the upward and downward thermal cycles hundreds of times.
Continuous furnaces are currently used in processing electronic components for such steps as depositing films, sintering contacts and heating to reflow solder for packaging semiconductor chips. In these processes an important goal is to avoid thermally stressing the components so modest rates of change of temperature are used.
In so far as the Applicants are aware, the prior art has failed to suggest the use of a continuous, belt driven furnace for the stressing complex electronic parts. This is due in part to the capabilities of batch ovens.
Batch ovens are typically of small size, relatively low cost and have higher thermal and gas efficiency when compared to continuous furnaces. In comparing the throughput of batch and continuous flow furnaces, considerations including the size and shape of the components and the time for the thermal process determine the relative advantage. For example, semiconductor wafers and modules can be arranged in a horizontal or vertical stack in a batch oven, each taking up a small volume within the oven; in a continuous furnace an equal number of wafers or modules must be spread flat on the belt, taking up a large furnace area. Thus, batch ovens can exceed the throughput of larger continuous furnaces, giving batch ovens significant floor space, capital, and process cost advantages for low thermal mass components.
Further, there are several problems which are encountered with continuous furnaces, the main disadvantage of standard continuous furnaces for thermal stressing electronic components is that continuous furnaces can provide only a small number of heating and cooling cycles, most preferably one cycle. For more than one cycle, a sequence of furnaces or means to repeatedly bring the components back to the beginning of the furnace must be implemented. These substantially increase capital costs and floor space or degrade throughput. There are several additional problems that must be overcome to thermally stress electronic components in a continuous furnace:
1. The temperature a thermally massive component experiences depends on what precedes and follows it on the belt.
2. To rapidly heat and cool components, the belt must move them through the furnace fast; the demands on the furnace to provide heat to the component or sink heat from the component increase with the rate of temperature change required and thermal mass of the components.
3. Industry and military thermal stress standards call for thermal stresses to be experienced by the component as a whole. There is usually no requirement for the part to withstand large thermal gradients from front to back. In continuous furnaces, components experience these unwanted thermal gradients across their length as they move from one temperature zone to another. The front edge of the component may begin cooling while the back edge is still in a hot zone. If this kind of bulk stress is unlikely to be experienced during the normal life of a component the stress might make components fail that do not have substantial manufacturing defects, unnecessarily reducing yield and adding cost. This problem is accentuated for high thermal mass components because the furnace needs to transfer heat at a faster rate.
4. Means for rapid and controlled heating are available, but means for equally rapid and controlled cooling to low temperature have been unavailable in continuous furnaces.
Because of the advantages of batch ovens and the disadvantages of continuous furnaces, the prior art did not anticipate the use of continuous furnaces for thermally stressing electronic components. Continuous furnaces do have the advantages of a constant product flow, low operator interface and individual processing of each component with no process wait time. For these reasons a continuous furnace for thermal stressing of large components is desirable.